The ability of fused silica to transmit ultraviolet (UV) radiation has caused this synthetic material to receive increasing attention recently. Fused silica lenses have found a variety of uses in applications requiring transmission of UV radiation at wavelengths below 300 nm. and with an intensity of 100 mJ/cm.sup.2 /pulse or greater.
One such application of particular interest is microlithography systems. These systems are used in the production of integrated circuits where the move is toward ever smaller sizes. This move is expected to require microlithographic systems employing pulsed, KrF excimer lasers emitting 248 nm. radiation. Such systems require 20-30 fused silica lenses commonly referred to as stepper and scanner lenses.
Earlier studies showed that synthetic fused silica, when exposed to prolonged 248 nm. radiation, underwent increased UV absorption damage, increased red (650 nm.) fluorescence and compaction (densification) of the glass. The absorption damage is observed as absorption bands centered at 215 and 260 nms. The 215 nm. band is prevalent in all fused silicas. The tails of these bands tend to create a significant absorption of the 248 nm. wavelength radiation from a KrF laser, or of the 193 nm. wavelength radiation from an ArF laser.
At low exposures, the 215 nm. absorption and the compaction increase with the number of pulses and with laser intensity. The 215 nm. absorption relaxes partially when exposure is interrupted, but returns with renewed exposure. At higher exposure levels, the 215 nm. absorption increases rapidly and becomes permanent, that is, it does not relax when exposure stops. This transition is accompanied by a marked increase in absorption at 260 nm., in the red fluorescence, and in sample temperatures.
The transition has been termed "sudden absorption transition (SAT)" in a publication by D. Krajnovich, I. K. Pour, A. C. Tam, W. Lueng and M. Kulkarmi entitled "Sudden Onset of Strong Absorption Followed by Forced Recovery in KrF Irradiated Fused Silica", OPTICS LETTERS, Vol. 18, No. 6, pp. 453-455, Mar. 15, 1993.
It is a purpose of the present invention to provide a method of producing a fused silica lens that transmits ultraviolet radiation at wavelengths below 300 nm. without undergoing a marked absorption transition or an increased red fluorescence, and that does not require forced cooling of the lens.
It is another purpose to provide a method of producing a fused silica lens that incurs controlled optical damage when transmitting ultraviolet radiation at wavelengths below 300 nm., whereby the absorption transition effect is not encountered and the damage stabilizes without further increase at a certain level.
It is a further purpose to provide a method of producing improved fused silica lenses for use in transmitting radiation from a KrF or a ArF laser.
It is another purpose to provide a method of producing improved fused silica, stepper and scanner lenses for use in a microlithography system.
It is a still further purpose to provide a method of producing a fused silica that transmits in excess of ten million pulses of radiation having a wavelength below 300 nm. and an intensity of at least 100 mJ/cm.sup.2 /pulse without undergoing a marked absorption transition.
It is also a purpose to provide improved, fused silica lenses produced by the improved method.